FIGS. 1A and 1B are schematic diagrams respectively showing a conventional VSSB coupling device 10 and a conventional word line driver 20. The VSSB coupling device 10 and the word line driver 20 may be used in a conventional word line driver system (not shown). Such a driver system typically utilizes a plurality of VSSB coupling devices 10 and a plurality of word line drivers 20. The VSSB coupling devices 10 and word line drivers 20 are arranged so that each VSSB coupling device supplies VSSB voltages to a select group of the word line drivers 20.
The VSSB coupling device 10 of FIG. 1A includes P-channel transistors 101, 102, and 103, an N-channel transistor 104 and inverters 111, 112, 113, and 114. The VSSB coupling device 10 is capable of coupling a negative boosted voltage VBB (e.g. −0.35 volts) or a ground supply voltage VSS (e.g., 0 volts) to its associated group of word line drivers 20.
The word line driver 20 of FIG. 1B includes P-channel transistors 201 and 202 and N-channel transistors 203, 204 and 205. The sources of N-channel transistors 204 and 205 are separately coupled to VSS and the source of N-channel transistor 203 is coupled to VSSB input terminal 207. The P-channel transistor 201 and N-channel transistor 203 form a last stage of the word line driver 20. The last stage ultimately controls access to memory cells (e.g. DRAM, SRAM, etc.) coupled to a word line WL by deactivating and activating the word line WL. To deactivate the word line WL, the P-channel transistor 201 is turned on, thereby pulling the word line WL up to a boosted positive word line voltage VPP (e.g., 1.5 volts) applied at VPP input terminal 206. To activate word line WL, the N-channel transistor 203 is turned on, thereby pulling down word line WL to the boosted negative voltage VBB applied at the VSSB terminal 207.
When one of the word lines WL controlled by the group of the word line drivers 20 is selected for access, the gates of the N-channel transistors 203 of the unselected word line drivers 20 in the group are biased at VSS and their corresponding sources are coupled to VBB. Hence in each of the unselected word line drivers 20, a significant sub-threshold current, caused by a positive cross voltage VGS (equal to VSS-VBB) between the gate and source of the N-channel transistor 203, will form a current path from VPP to VBB. This results in a voltage level drop in VPP for the unselected word lines WL controlled by the unselected word line drivers 20 and voltage level shallow in VBB for the selected word line.
In a standby state, when none of the word lines in the group of word lines have been selected, the P-channel transistors 201 of the word line drivers 20 are turned on and the N-channel transistors 203 of the word line drivers 20 are turned off, as shown in FIG. 1C (shows only the last stage of one of the word line drivers 20). The turned on P-channel transistors 201 couple their word lines WL to receive the boosted positive voltage VPP and the associated VSSB coupling device 10 is switched to couple or output the ground supply voltage VSS at VSSB output terminal 109 to the word line drivers 20. Thus, node N1 is maintained at VBB by P-channel transistor 102 and output terminal 109 is coupled to VSS by N-channel transistor 104, thereby creating a non-zero gate-source cross voltage VGS at P-channel transistor 101, which causes a leakage current from VSS to VBB.
In addition, in the standby state, a high cross voltage VDS (equal to VPP-VSS) exists between the source and drain of the last stage N-channel transistor 203 of each of the word line driver 20. This cross voltage causes significant sub-threshold channel leakage, possibly pulling down the VPP level, if VPP pump driving capability is poor. Further, the large number of the word line drivers 20 in each group (e.g., 32 word line drivers) in the standby state consume excessive power. These conditions are aggravated during high temperature operation.
Moreover, when one of the word lines WL is selected for access, the gate of the N-channel transistor 104 of the associated VSSB coupling device 10 is biased at VSS. Hence, a significant sub-threshold current, caused by a positive gate-source cross voltage VGS where VGS is the cross bias between node N2 and VSSB in FIG. 1A according to the definition of source node at which majority carriers emit. Accordingly, VGS=VSS−VBB>0 for selected word line condition resulting in a voltage level drop in VBB in the selected word line WL. Consequently, the selected (activated) word line may not reach its desired VBB level. Since VBB is a negative voltage level, a weakly conducted N-channel transistor 104 will make VBB voltage level “shallower” or closer to the ground supply voltage VSS through the N-channel transistor 104.
Accordingly, there is a need for a word line control circuit that maintains boosted voltages and has significantly reduced leakage currents and power consumption in the active and standby modes.